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Annals of Mathematics Existence and minimizing properties of retrograde orbits to the three-body problem with various choices of masses By Kuo-Chang Chen Annals of Mathematics, 167 (2008), 325–348 Existence and minimizing properties of retrograde orbits to the three-body problem with various choices of masses By Kuo-Chang Chen Abstract Poincar´e made the first attempt in 1896 on applying variational calculus to the three-body problem and observed that collision orbits do not necessarily have higher values of action than classical solutions. Little progress had been made on resolving this difficulty until a recent breakthrough by Chenciner and Montgomery. Afterward, variational methods were successfully applied to the N-body problem to construct new classes of solutions. In order to avoid collisions, the problem is confined to symmetric path spaces and all new planar solutions were constructed under the assumption that some masses are equal. A question for the variational approach on planar problems naturally arises: Are minimizing methods useful only when some masses are identical? This article addresses this question for the three-body problem. For var- ious choices of masses, it is proved that there exist infinitely many solutions with a certain topological type, called retrograde orbits, that minimize the action functional on certain path spaces. Cases covered in our work include triple stars in retrograde motions, double stars with one outer planet, and some double stars with one planet orbiting around one primary mass. Our results largely complement the classical results by the Poincar´e continuation method and Conley’s geometric approach. 1. Introduction Periodic and quasi-periodic solutions to the Newtonian three-body prob- lem have been extensively studied for centuries. Until today, in general it is still a difficult task to prove the existence of solutions with prescribed topological types and masses. Calculus of variations, in spite of its long history, should be considered a relatively new approach to the three-body problem. In 1896 Poincar´e [23] made the first attempt to utilize minimizing methods to obtain solutions for the three-body problem, but found out the discouraging fact that existence of collisions does not necessarily cause a significant increment in the value of 326 KUO-CHANG CHEN the action functional. As a result solutions were obtained only for the strong- force potential, instead of the Newtonian case. In 1977 Gordon [13] proved a minimizing property for elliptical Keplerian orbits, including the degenerate case – collision-ejection orbit. It turns out that the actions of these orbits over one period depend only on the masses and the period, not on eccentricity. From this point of view the collision-ejection orbits and other elliptical orbits are not distinguishable. A common doubt at the time is: Are minimizing methods useful for the N-body problem? Concerning this question, Chenciner- Venturelli [8] constructed the “hip-hop” orbit for the four-body problem with equal masses and, a few months later, Chenciner-Montgomery [7] constructed the celebrated figure-8 orbit for the three-body problem with equal masses, a solution numerically discovered in [20]. Afterward, Marchal [16] found a class of solutions related to the figure-8 orbit and made important progress on excluding collision paths [17], [5]. Inspired by the discovery of the figure- 8 orbit, a large number of new solutions [2], [3], [4], [11], [26] were proved to exist by variational methods. These discoveries attract much attention not only because they are not covered by classical approaches, but also due to the amusing symmetries they exhibit. On the other hand, these orbits were constructed under the assumption that some masses are equal. Except a class of nonplanar solutions constructed by varying planar relative equilibria in a direction perpendicular to the plane (see Chenciner [5], [6]), among the discoveries for the N-body problem, none of the new solutions constructed by variational methods can totally discard this constraint. A question for the variational approach, especially on planar problems, naturally arises: Are minimizing methods useful only when some masses are identical? This article is concerned with variational methods on the existence of certain types of solutions to the planar three-body problem with various choices of masses. There is a natural way of classifying orbits by their topological types in the configuration space. From the terminology normally used in lunar theory, we call a solution retrograde if its homotopy type in the configuration space (with collision set removed) is the same as those retrograde orbits in the lunar theory. Detailed descriptions are left to Section 2 and 3. Our main theorem (Theorem 1) shows the existence of many periodic and quasi-periodic retrograde solutions to the three-body problem provided the mass ratios fall inside the white regions in Figure 1. The method used is a variational approach with a mixture of topological and symmetry constraints. The advantage of our approach, as Figure 1 indicates, is that it applies to a wide range of masses. In sharp contrast with the results obtained from the classical Poincar´e continuation method [22] (see [24], [18] and references therein) and Conley’s geometric approach [9], [10], our main theorem does not apply to Hill’s lu- nar theory and many satellite orbits, both of which treat the case with one dominant mass. It is worth mentioning that Hill’s lunar theory can also be EXISTENCE AND MINIMIZING PROPERTIES OF RETROGRADE ORBITS 327 0 1 2 3 4 2.50 0.62 m 1 m 3 0 m 1 m 3 m 2 m 3 m 2 m 3 3421 200 20050 50 150 100 100 150 Figure 1: Admissible mass ratios (the white region) for the main theorem. analysized by variational methods; see Arioli-Gazzola-Terracini [1]. Cases we are able to cover include retrograde triple stars, double stars with one outer planet, and some double stars with one planet orbiting around one primary mass. See Section 2 and Figure 3 for details. Moreover, due to the minimizing properties the orbits we obtained do not contain tight binaries, and there are periodic ones with very short periods in the sense that the prime periods are small integral multiples of their prime relative periods. Classical approaches normally produce orbits with very long periods. 2. The Main Theorem The planar three-body problem concerns the motion of three masses m 1 , m 2 , m 3 > 0 moving in the complex plane C in accordance with Newton’s law of gravitation: m k ¨x k = ∂ ∂x k U(x),k=1, 2, 3(1) where x =(x 1 ,x 2 ,x 3 ), x k ∈ C is the position of m k , and U(x)= m 1 m 2 |x 1 − x 2 | + m 2 m 3 |x 2 − x 3 | + m 1 m 3 |x 3 − x 1 | , is the potential energy (negative Newtonian potential). The kinetic energy is given by K(˙x)= 1 2  m 1 | ˙x 1 | 2 + m 2 | ˙x 2 | 2 + m 3 | ˙x 3 | 2  . There is no loss of generality to assume that the mass center is at the origin; that is, assuming x stays inside the configuration space: V := {x ∈ C 3 : m 1 x 1 + m 2 x 2 + m 3 x 3 =0} . 328 KUO-CHANG CHEN collin ea r a cu te o b tu s e ob tuse 2 1 isosceles collinear double collision equilateral triangle 2 31 a cu te 3 1 3 1 3 2 3 2 3 1 2 1 2 Λ 3 Λ 2 Λ 1 ˜α φ Figure 2: The unit shape sphere. A preferred way of parametrizing V is to use Jacobi’s coordinates: (z 1 ,z 2 ):=   M 1 (x 2 − x 1 ),  M 2 (x 3 − ˆx 12 )  , where M 1 = m 1 m 2 m 1 +m 2 , M 2 = (m 1 +m 2 )m 3 m 1 +m 2 +m 3 , and ˆx 12 = 1 m 1 +m 2 (m 1 x 1 + m 2 x 2 )is the mass center of the binary {x 1 ,x 2 }. The reduced configuration space ˜ V is obtained by quotient out from V the rotational symmetry given by the SO(2)- action: e iθ · (z 1 ,z 2 )=(e iθ z 1 ,e iθ z 2 ). The identification ˜ V = V/SO(2) is via the Hopf map (u 1 ,u 2 ,u 3 ):=(|z 1 | 2 −|z 2 | 2 , 2 Re(¯z 1 z 2 ), 2 Im(¯z 1 z 2 )) .(2) Each single point in ˜ V represents a congruence class of triangles formed by the three mass points, and each point on its unit sphere {|u| 2 =1}, called the unit shape sphere, represents a similarity class of triangles. The signed area of the triangle is given by 1 2 u 3 . Figure 2, due to Moeckel [19], relates the configurations of the three bodies with points on the unit shape sphere. In the figure Λ j represents isosceles tri- angles with jth mass equally distant from the other two. The equator (u 3 =0) represents collinear configurations. On the upper hemisphere (u 3 > 0), trian- gles with vertices {x 1 ,x 2 ,x 3 } are positively oriented; on the lower hemisphere they are negatively oriented. The poles correspond to equilateral triangles. Let Δ := {x ∈ C 3 : x i = x j for some i = j} be the variety of collision configurations. It is invariant under rotations and its projection ˜ Δin ˜ V is the union of three lines emanating from the origin (the triple collision). Each line represents a similarity class of one type of double collision. Let S 3 be the unit EXISTENCE AND MINIMIZING PROPERTIES OF RETROGRADE ORBITS 329 sphere in V and S 2 be the unit shape sphere. The Hopf fibration (2) renders S 3 \ Δ the structure of an SO(2)-bundle over S 2 \ ˜ Δ, whose fundamental group is a free group with two generators. For φ>0, let α φ be the following loop in V \ Δ: α φ (t):=e φti  m 3 (M − m 2 ) − m 2 Me −2πti ,(3) m 3 (M + m 1 )+m 1 Me −2πti , −(m 1 + m 2 )M  , where M = m 1 + m 2 + m 3 is the total mass. The homotopy class of the projection ˜α φ of α φ in ˜ V \ ˜ Δ over t ∈ [0, 1] is one of the two generators for π 1 (S 2 \ ˜ Δ). The left side of Figure 2 depicts the path ˜α φ over t ∈ [0, 1]. A solution x of (1) is called relative periodic if its projection ˜x in the reduced configuration space ˜ V is periodic. The prime relative period of x is the prime period of ˜x. Our major result concerns the existence of relative periodic solutions to the three-body problem that are homotopic to α φ in V \Δ respecting the rotation and reflection symmetry of α φ . A precise description is given in (9). These types of solutions, called retrograde orbits, are of special importance in the three-body problem. When 0 <m 1 ,m 2  m 3 , the search for this type of solutions is an important problem in lunar theory. A typical example is the system Sun-Jupiter-Asteroid. When 0 <m 3  m 2 ,m 1 , these types of solutions are sometimes called satellite orbits or comet orbits. If all masses are comparable in size and none of them stay far from the other two, then the system forms a triple star or triple planet. Another interesting case is 0 <m 2  m 1 ,m 3 . The binary m 1 , m 3 form a double star (or double planet) and m 2 is a planet (or satellite) orbiting around m 1 . There is no evident borderline between these categories. The dash lines in Figure 3 make a rough sketch of the borders between them. There is no loss of generality in assuming m 3 = 1. Let M = m 1 + m 2 +1 be the total mass. Define functions J :[0, 1) → R + and F, G : R 2 + → R by J(s):=  1 0 1 |1 − se 2πti | dt ,(4) F (m 1 ,m 2 ):= 3 2  2 2/3 −1 max{m i } +1−  M m 1 +m 2  1 3  ,(5) G(m 1 ,m 2 ):= 1 m 1  J  m 1 M 1/3 (m 1 +m 2 ) 2/3  − 1  (6) + 1 m 2  J  m 2 M 1/3 (m 1 +m 2 ) 2/3  − 1  . The following is our main theorem. Theorem 1. Let m 3 =1,M = m 1 + m 2 +1 be the total mass, and let F , G be as in (5), (6). Then the three-body problem (1) has infinitely many 330 KUO-CHANG CHEN periodic and quasi-periodic retrograde orbits provided F (m 1 ,m 2 ) >G(m 1 ,m 2 ) .(7) Furthermore, there exists a periodic retrograde orbit whose prime period is twice its prime relative period. Theorem 1 applies to the complement of the shaded region in Figure 3. Following from a minimizing property described in Section 3, orbits given by Theorem 1 do not possess tight binaries. In Section 6 we will explain this and demonstrate a more general theorem. Classical results on retrograde orbits treat the case with one tight binary or with one dominant mass, including Hill’s lunar theory and some satellite orbits. From this point of view Theorem 1 largely complements classical results. 0 1 0.62 m 1 ··· Triple Star in retrograde motion m 2  1  1 . . . Double Star with one planet 1 A star with two planets Double Star with one outer planet or comet Lunar orbit (satellite orbits) 2 2 2.50 orbiting around one primary mass Figure 3: Theorem 1 applies to the complement of the shaded region. 3. A minimizing problem In this section we set up a variational problem for which minimizers exist and which solves (1) with the claimed properties in Theorem 1. Equation (1) and following are the Euler-Lagrange equations for the action functional A : H 1 loc (R,V) → R ∪{+∞} defined by A(x):=  1 0 K(˙x)+U(x) dt . By choosing a sequence of motionless paths with greater and greater mutual distances, it is easy to see that the infimum of A on H 1 loc (R,V) is zero, which EXISTENCE AND MINIMIZING PROPERTIES OF RETROGRADE ORBITS 331 is not attained. To ensure that the minimizing problem is solvable, we select the following ground space: H φ := {x ∈ H 1 loc (R,V): x(t)=e −φi x(t +1)} , where φ ∈ (0,π] is some fixed constant. Any path x in H φ satisfies x(0),x(1) = cos φ|x(0)|·|x(1)|. Here ·, · represents the standard scalar product on (R 2 ) 3 . From this condi- tion, the action functional A restricted to H φ is coercive (see [3, Prop. 2], for instance). By using Fatou’s lemma and the fact that any norm is weakly se- quentially lower semicontinuous, it is an easy exercise to show that A is weakly sequentially lower semicontinuous on H φ . Following a standard argument in the calculus of variations, the action functional A attains its infimum on H φ . Although it may appear as an easy fact, let us remark here that collision- free critical points of A restricted to H φ are classical solutions to (1). If H ∗ φ is the space H φ except that the configuration space V is replaced by (R 2 ) 3 , then on H ∗ φ the fundamental lemmas for the calculus of variations are clearly applicable. Now if x is a collision-free critical point of A restricted to H φ , from the first variation of A constrained to H φ ,atx we have 0=δ h A(x)=−  1 0 3  k=1  m k ¨x k − ∂U ∂x k  · h k dt for any h =(h 1 ,h 2 ,h 3 ) ∈ C ∞ 0 ([0, 1],V). Let y k = m k ¨x k − ∂U ∂x k , then (y 1 (t),y 2 (t),y 3 (t)) ∈ V ⊥ for any t. A basis for the subspace V ⊥ of (R 2 ) 3 is {(m 1 , 0,m 2 , 0,m 3 , 0), (0,m 1 , 0,m 2 , 0,m 3 )}. Therefore y i (t)=m i α(t) for some α :[0, 1] → R 2 and for each i. It can be easily verified that  3 k=1 y k (t) = 0, that is (m 1 + m 2 + m 3 )α(t) = 0. Then α and hence every y i is identically zero. This proves that x is indeed a classical solution of (1). The conventional definition of inner product on the Sobolev space H 1 ([0, 1],V) defines an inner product on H φ as well: x, y φ :=  1 0 x(t),y(t) +  ˙x(t), ˙y(t) dt . Critical points of A on H φ are critical points of A on H 1 ([0, 1],V). One can easily verify that, for any x ∈ H φ and τ ∈ R, A(x)=  1+τ τ K(˙x)+U(x) dt , x, y φ =  1+τ τ x(t),y(t) +  ˙x(t), ˙y(t) dt . 332 KUO-CHANG CHEN From these observations, any critical point x of A on H φ is a solution of (1), but possibly with collisions. If we can show that x has no collision on [0, 1), then there is no collision at all and x indeed solves (1) for any t ∈ R. Moreover, x is periodic if φ π is rational; it is quasi-periodic if φ π is irrational. Consider a linear transformation g on H φ defined by (g · x)(t):= x(−t) .(8) The space of g-invariant paths in H φ is denoted by H g φ . That is, H g φ := {x ∈ H φ : g · x = x} . Observe that g is an isometry of order 2, and the action functional A defined on H φ is g-invariant. By Palais’ principle of symmetric criticality [21], any collision-free critical point of A while restricted to H g φ is also a collision-free critical point of A on H φ , and hence solves (1). Let α φ be as in (3). The space X φ of retrograde paths in H g φ is defined as the path-component of collision-free paths in H g φ containing α φ . In other words, X φ :=  x ∈ H g φ : x(t) ∈ Δ for any t, x is homotopic to α φ in V \ Δ within the class of collision-free paths in H g φ  .(9) The set X φ is an open subset of H g φ . Therefore, critical points of A in X φ ,if they exist, are retrograde orbits. Now we consider the following minimizing problem: inf x∈X φ A(x) .(10) As noted before, the action functional A is coercive and hence attains its infimum on the weak closure of X φ . The boundary ∂X φ of X φ consists of paths in H g φ that have nonempty intersection with the collision set Δ. The next two sections are devoted to proving the inequality inf x∈X φ A(x) < inf x∈∂X φ A(x) for φ ∈ (0,π] sufficiently close to π, under the assumptions in Theorem 1. 4. Upper bound estimates for the action functional A This section is devoted to providing an upper bound estimate for (10). Assume m 3 =1,φ ∈ (0,π], and M = m 1 + m 2 + 1. Let Q(t):= 1 (Mφ) 2/3 e φti , R(t):= 1 (m 1 + m 2 ) 2/3 (2π − φ) 2/3 e (φ−2π)ti , EXISTENCE AND MINIMIZING PROPERTIES OF RETROGRADE ORBITS 333 and x (φ) (t)=(x (φ) 1 ,x (φ) 2 ,x (φ) 3 ) := (Q(t) − m 2 R(t),Q(t)+m 1 R(t), − (m 1 + m 2 ) Q(t)) . It is routine to verify that x (φ) ∈X φ . See Figure 4 for the retrograde path x (φ) . 2 Q(t) 3 t =0 t = 1 2 1 2 1 3 φ/2 Figure 4: The retrograde path x (φ) . The calculation for K(˙x (φ) ) is simple: | ˙x (φ) 1 | 2 = φ 2/3 M 4/3 + m 2 2 (2π − φ) 2/3 (m 1 + m 2 ) 4/3 +2m 2 φ 1/3 (2π − φ) 1/3 M 2/3 (m 1 + m 2 ) 2/3 cos(2πt) | ˙x (φ) 2 | 2 = φ 2/3 M 4/3 + m 2 1 (2π − φ) 2/3 (m 1 + m 2 ) 4/3 − 2m 1 φ 1/3 (2π − φ) 1/3 M 2/3 (m 1 + m 2 ) 2/3 cos(2πt) | ˙x (φ) 3 | 2 =(m 1 + m 2 ) 2 φ 2/3 M 4/3 K(˙x (φ) )= 1 2  (m 1 + m 2 ) φ 2/3 M 1/3 + m 1 m 2 (2π − φ) 2/3 (m 1 + m 2 ) 1/3  . Note that K(˙x (φ) ) is independent of time. Define ξ = ξ(m 1 ,m 2 ,φ):= 1 M 1/3 (m 1 + m 2 ) 2/3  φ 2π − φ  2 3 ,(11) ξ π := ξ(m 1 ,m 2 ,π)= 1 M 1/3 (m 1 + m 2 ) 2/3 .(12) Let J(s) be as in (4). In terms of J and ξ, the contribution of U(x (φ) )tothe total action can be written [...]... by Lemma 2 and Lemma 4, inf A(x) < inf A(x) x∈Xπ x∈∂Xπ By continuity of the bounds in Lemmas 2 and 4 with respect to φ, there is some > 0 such that inf A(x) < inf A(x) x∈Xφ x∈∂Xφ for any φ ∈ (π − , π] This proves the existence of infinitely many periodic and quasi-periodic retrograde orbits for the three-body problem (1) under the assumption (7) By the construction of Xφ , the prime period of any minimizer... that solutions given by Theorem 1 do not possess tight binaries This can be seen from the following generalization of Theorem 1 Theorem 5 Let m3 = 1, M = m1 + m2 + 1 be the total mass, and let J, Xφ , ξ be as in (4), (9), (11) 339 EXISTENCE AND MINIMIZING PROPERTIES OF RETROGRADE ORBITS (a) Given any φ ∈ (0, π], the three-body problem (1) has a retrograde solution that minimize the action functional... conic section, ˙ ˙ there are constants p > 0, e ≥ 0, θ0 ∈ [0, 2π) such that p = 1 + e cos(θ − θ0 ) r Differentiating the identity with respect to t = 0 and T , this yields ˙ ˙ −e sin(−θ0 ) · θ(0) = 0 = −e sin(φ − θ0 ) · θ(T ) 337 EXISTENCE AND MINIMIZING PROPERTIES OF RETROGRADE ORBITS The only possibility is e = 0 because φ ∈ (0, π) and the angular momentum is nonzero This shows the minimizing orbit... crucial If we use, for instance, the na¨ estimate J(s) ≤ 1−s from ıve the definition of J(s), then the regions of admissible masses in Figure 5 would completely diminish 3 2.5 2 1.5 1 0.5 0 0.2 0.4 0.6 0.8 1 Figure 10: The graph of the function J(s) The function J(s) can be viewed as the potential at (1, 0) ∈ R2 of a circular ring centered at the origin with radius s and uniform density Moreover, 1 J(s)... π, 2π 3 and for certain masses Acknowledgement I am most grateful to Rick Moeckel, Alain Chenciner, and the referees for valuable comments Many thanks to Don Wang and Maciej Wojtkowski for enlightening conversations and their hospitality during my visit to the University of Arizona The research work is partly supported by the National Science Council and the National Center for Theoretical Sciences in... Arioli, F Gazzola, and S Terracini, Minimization properties of Hill’s orbits and applications to some N -body problems, Ann Inst H Poincar´ Anal Non Lin´aire 17 e e (2000), 617–650 [2] K.-C Chen, Action -minimizing orbits in the parallelogram four-body problem with equal masses, Arch Ration Mech Anal 158 (2001), 293–318 [3] ——— , Binary decompositions for the planar N -body problem and symmetric periodic... solutions of the n-body problem, Proc of NDDS Conference (Kyoto, 2002); Available at http://www.imcce.fr/Equipes/ ASD/person/chenciner/chen preprint.html [7] A Chenciner and R Montgomery, A remarkable periodic solution of the three-body problem in the case of equal masses, Ann of Math 152 (2000), 881–901 [8] A Chenciner and A Venturelli, Minima de l’int´grale d’action du Probl`me newe e tonien de 4... and S Terracini, On the existence of collisionless equivariant minimizers for the classical n-body problem, Invent Math 155 (2004), 305–362 [12] C F Gauss, Werke, Band III (1818), 331–355 [13] W Gordon, A minimizing property of Keplerian orbits, Amer J Math 99 (1977), 961–971 348 KUO-CHANG CHEN ´ [14] M Henon, A family of periodic solutions of the planar three-body problem, and their stability, Celestial... 267–285 [15] O D Kellogg, Foundations of potential theory, Die Grundlehren der Mathematischen Wissenschaften, Band 31, Springer-Verlag, New York, 1967 [16] C Marchal, The family P12 of the three-body problem the simplest family of periodic orbits, with twelve symmetries per period, Celestial Mech Dynam Astronomy 78 (2000), 279–298 [17] ——— , How the method of minimization of action avoids singularities,... large proportion of the star systems in the cosmos are double stars, there are good chances such wagging planets do exist somewhere 345 EXISTENCE AND MINIMIZING PROPERTIES OF RETROGRADE ORBITS 3 3 2 2 1 1 0 0 −1 −1 −2 −2 −3 −3 −2 −1 0 1 2 3 −3 −3 −2 −1 0 1 2 3 Figure 9: Double stars with wagging planets Appendix: Some properties of J(s) 4s (1+s)2 , Let J(s) be as in (4) In terms of a power series . Annals of Mathematics Existence and minimizing properties of retrograde orbits to the three-body problem with various choices of masses By Kuo-Chang Chen Annals of Mathematics,. 167 (2008), 325–348 Existence and minimizing properties of retrograde orbits to the three-body problem with various choices of masses By Kuo-Chang Chen Abstract Poincar´e made the first attempt in. some masses are identical? This article is concerned with variational methods on the existence of certain types of solutions to the planar three-body problem with various choices of masses. There